many thangs go as x=xstart*e^-kt, t=time yunit such as yeers
Yu need tu dind a value for k for wotever yu werk with
A nu kar in US drop tu half its nu value in bout 3 yeers
kall this the "half life"
If yuze equashun, x/xstart=e^-it & evaluate it at 3 yeer=1/2, yu get
e^-3k=0.5
take ln & yu get -3k=ln(0.5)
or k=-(1/3)ln(0.5)
ln(0.5)=-0.6931471805599453
so k=+0.23104906018665